FI, like CI, can produce molecular ions for some volatile compounds that do not give molecular ions by EI. FI produces dominant molecular ions with little or no fragmentation. FI utilizes 10-micron diameter tungsten emitter wires on which carbon whiskers, or dendrites, have been grown. A high electric field gradient (10⁸ V/cm) at the tips of the whiskers produces ionization by quantum mechanical tunneling of electrons. FI is also used for performing isotope ratio measurements on samples that either give small molecular ions or large (M-H)⁺ ions in EI. There are three practical differences between CI and FI: there is less fragmentation in FI, there is no high-resolution FI, and FI is less sensitive. Sensitivity is not an issue unless there is an extremely small amount of sample. FI can be performed by direct probe and GC/MS.

Low resolution FI Instrument: 70-VSE Sample size: 1-2 mg

Output for FI is in the form of a bar plot and/or mass list.

Isotope ratio Instrument: 70-VSE Sample size: 1-2 mg

FI isotope ratio measurements are performed by scanning a small region around the molecular ion region in continuum mode. This technique is used to determine the incorporation of ²H, ¹³C, ¹⁸O, etc. You will receive a computer printout listing the area of each peak, which is proportional to the amount present. Most researchers have programs on their own computers to calculate isotopic enrichment from these areas, but we can do this for you upon request. All isotope ratio work requires both a standard as well as unknown samples. Both isotope-enriched and standard samples should be submitted at the same time to insure that instrumental factors will not affect the measurements. The 70-VSE is used for isotope ratio measurements by direct probe EI or CI, or by GC/MS with EI, CI, or FI ionizat Measurements can also be performed by ESI or FAB if necessary. Please consult with the associate director or your group expert about isotope ratio problems.